The present application relates, generally, to light amplification and laser devices, and, more particularly, to light amplification and laser devices that utilize a thulium doped modified silicate optical fiber.
The extraordinary advancement of wide area networking services, e.g., the Internet, over the past several years has been enabled by the confluence of two key technologies, i.e., the erbium doped optical fiber amplifier, EDFA, and wavelength division multiplexing, WDM. Since the discovery by Townsend and Payne in the late 1980's of a method for fabricating high quality rare-earth doped silica fibers, much work has centered on the development of and the exploitation of the EDFA. The typical EDFA consists of Er3+ doped into an alumino-silicate glass optical fiber. The developments have revolutionized the telecommunications industry as EDFA has replaced electronic repeaters in fiber based networks. The EDFA coupled with the development of WDM technology has allowed for the engineering of high bandwidth optical systems in the region of 1525 to 1570 nm. This is within the “low-loss” or “third” optical fiber telecommunications window. The low-loss window is the range 1420 nm to 1650 nm where the attenuation per unit length for silica optical fiber is near its minimum, e.g., <0.35 dB/km. The C-band 1525 to 1585 nm, and L-band, 1585 to 1650 nm, are each covered by the EDFA, but it is apparent that these two bands represent a portion of the low-loss region for silica but not the total. Due to the fortunate coincidence of the Er3+ gain transition with the low-loss window, the EDFA has come to be extensively used in optical fiber telecommunications systems. The EDFA has also enabled the transmission of enormous quantities of data via WDM, that is, by providing gain simultaneously for multiple data transmission channels at different wavelengths within the bandwidth of the EDFA. To date no practical amplifier has been demonstrated for wavelengths of <1520 nm, so that fully half of the low-loss window bandwidth is unused.
There is a desire for the development of the S-band amplifier. This requires that the rare-earth ion with an appropriate transition have fluorescence in the region of approximately 1450 to 1520 nm. Tm3+ has the necessary fluorescence. The relevant transition is 3H4 to 3F4, which fluoresces at 1430-1500 nm. In the absence of nonradiative quenching, the lifetime of the upper level, 3H4, is expected to be approximately, 1.5 ms; this is observed for Tm3+ in low phonon energy fluorozirconate glasses. However, the energy separation between 3H4 level and the next lower level, 3H5, is sufficiently small, 4400 cm−1, that the upper level is substantially quenched by multiphonon processes in high-phonon energy glasses like the silicates. The lifetime has been measured as <20 μs in a pure silica host. Depletion of the upper state lifetime via nonradiative processes reduces the population available to provide gain on the transition of interest. While fiber amplifiers based on this transition have been demonstrated in fluorozirconate glasses, these have proved impractical due to various problems with the host material.
Thulium, Tm, has a 3H4 to 3F4 transition which provided amplification in the S-band wavelength range using a fluorozirconate host. This fluorozirconate material possesses properties that do not lend the material for use in lasers or in optical fibers. These materials are hygroscopic, prone to formation of micro-crystallites over time and have glass transition temperatures at about 400° C. which prevents fusion splicing to standard telecommunications-grade fibers. In the event these glasses are butt spliced they tend to become damaged with heavy pumping.
Although the fluoride and tellurite hosts doped with thulium offer high quantum efficiencies for the 1.47 μm transition, some of the material's properties are problematic with respect to making a practical device. Fluoride glasses are very difficult to fabricate into low-loss fiber due to a propensity towards crystallization and suffer from poor chemical durability. Tellurite glasses, although stable, have a high index of refraction and high thermal expansion, which complicates splicing into an all-optical system.
With the advent of new silica fibers with low-loss across the entire region of 1200 to 1600 nm, i.e., <0.35 dB/km, optical amplifiers that can potentially amplify at other wavelengths within this region are of increased importance.
Silica host materials do have both good chemical and mechanical properties, e.g., fusion splicing to the silicates, high mechanical strength, high glass transition temperature, and extremely low thermal expansion. However, doping silica materials with Tm3+ has low fluorescence and high phonon quenching and therefore is not practical for use in optical fiber systems.
However, a silica glass material doped with Tm3+, Ho3+, and Tm3+-sensitized-Ho3+ in which the material has reduction in the multiphonon quenching compared to the multiphonon quenching of pure silicates has recently been proposed. This material overcomes some of the difficulties with utilizing thulium discussed above. It would therefore be desirable to provide a device that amplifies light at wavelengths in the vicinity of 1420-1530 nm, using such a thulium doped silica-based optical fiber.